1. Field of the Invention
The present invention relates to a conveyance technique of a printing medium or the like.
2. Description of the Related Art
In recent years, a printing apparatus such as a copying machine or a printer is often used to print a photographic image. Especially, an inkjet printing apparatus can form an image of the same quality as a silver halide photo on the strength of reduction of the ink droplet size and improvement of image processing technologies.
Against the backdrop of the demand for higher image quality, a high accuracy is required to convey a printing medium. In particular, regarding a roller for conveying the printing medium, a very high accuracy is needed because the printing medium conveyance amount is almost proportional to the outer diameter of the roller. However, the accuracy of finishing of the roller is limited. Hence, there is a need of conveyance control capable of implementing a high conveyance accuracy regardless of a variation in the outer diameter of the roller or decentering of the roller.
In general, the main printing unit of the printing apparatus is formed from a printhead and a plurality of conveyance rollers provided on the upstream or downstream side of the printhead. In the printing apparatus having this arrangement, the conveyance amount upon switching the roller involved in conveyance is particularly problematic concerning the printing medium conveyance accuracy. For example, when switching from a state in which the printing medium is conveyed only by the conveyance roller on the upstream side to a state in which the printing medium is conveyed by two conveyance rollers on the upstream and downstream sides, the conveyance accuracy may lower due to the influence of the conveyance amount difference between the conveyance rollers. This degrades the image quality. To cope with this problem, Japanese Patent Laid-Open No. 4-148958 proposes a method of correcting the conveyance amount upon switching the conveyance state.
In the state in which the printing medium is conveyed by the two conveyance rollers on the upstream and downstream sides, loads act to uniform the conveyance amounts of the conveyance rollers. More specifically, forces in opposite directions are applied to the conveyance rollers through the printing medium. The forces cause the conveyance rollers to slip and make their conveyance amounts equal.
Examine this phenomenon in more detail. Because of the loads acting on the conveyance rollers, another phenomenon also takes place in which the conveyance rollers bend to themselves. Since this bending displaces the conveyance rollers sandwiching the printing medium, the position of the printing medium changes as well. This leads to a decrease in the conveyance accuracy.
Additionally, immediately after the switching of the conveyance state, the loads applied to the conveyance rollers fluctuate and then transit to a stable state. Japanese Patent Laid-Open No. 4-148958 pays no attention to the load fluctuation upon switching the conveyance state.